Electrical and Structural Characterization of a Low-Angle Tilt Grain Boundary in Iron-Doped Strontium Titanate

The atomistic structure and electrical properties of a symmetrical 5.4° [001] tilt grain boundary in Fe-doped SrTiO₃ have been investigated, respectively, by means of various transmission electron microscopy (TEM) techniques and impedance spectroscopy. In weak-beam dark-field images, the grain boundary is revealed to consist of a periodic array of dislocations; high-resolution TEM images show that the dislocation cores are separated by regions of strained lattice. The impedance response of the bicrystal has been measured in the frequency range 20 Hz ≤ f ≤ 10⁶ Hz as a function of temperature and oxygen partial pressure. The transport of charge across the array of dislocations that form the grain boundary is strongly hindered. Analysis of the impedance data in terms of a double-Schottky-barrier model yields a space-charge potential that exhibits a weak dependence on temperature and oxygen partial pressure and is ∼0.55 V in the investigated regime.     

Low-angle twist grain boundary in SrTiO3 fabricated by spark plasma sintering techniques

A SrTiO₃ bicrystal with a low-angle twist grain boundary was fabricated using the spark plasma sintering (SPS) instrument. The atomic and electronic structure of the grain-boundary core was characterized using scanning transmission electron microscopy techniques. It was determined that the boundary is comprised of 2 types of defects with distinct electronic structures: screw dislocations and dislocations with an [001] edge component. The dislocations with an [001] edge component dissociated into 2 partial dislocations, separated by a stacking fault consisting of 2 Ti–O layers. The screw dislocations are attributed to the twist component of the grain boundary, while dislocations with an [001] edge component are attributed to surface steps on the original (100) SrTiO₃ surfaces prior to diffusion bonding. The observed repeat distances between the dislocations with edge components along the grain-boundary plane are smaller than those discovered during traditional diffusion bonding experiments. The higher planar defect density observed in this study results partly from higher heating rates, lower processing temperatures, and shorter holding times during SPS processing.     

Subgrain Boundaries in MgO

The structure of subgrain boundaries in as-grown MgO was characterized by transmission electron microscopy. The types of subboundaries which occur quite frequently are (1) a pure tilt boundary consisting only of ½ [101]-type dislocations ( A set); (2) a tilt boundary consisting mainly of ½ [101] ( A set) and ½ [011] ( B set) dislocations; and (3) a mixed boundary with ½ [101] ( A set), ½ [011] ( B set), and ½ [     01] ( C set) dislocations. At the nodes the following dislocation reactions occur: ½ [011]+½ [10     ] =½ [110] and ½ [10     ]+½ [101] = [100]. The precipitation of impurities at the subboundaries and their role in pinning the subboundaries are discussed.     

Energy and Structure of (001) Coincident-Site Twist Boundaries and the Free (001) Surface in MgO: A Theoretical Study

A computer code is described which permits investigation of the energy and structure of coincident-site lattice (CSL) grain boundaries, stacking faults, and free surfaces in ionic crystals. The code uses computational techniques similar to those of Harwell's HADES code for the study of point defects in bulk ionic crystals. Two sets of short-range potentials are used to determine the energy and structure of the free (001) surface and of (001) CSL twist boundaries in MgO with values of Γ, the inverse density of CSL sites, ranging between Γ= 5 and Γ= 65. From comparison of the results obtained by means of the different potentials it is concluded that the Van der Waals attraction between oxygen ions on opposite sides of the interface is mainly responsible for the rather weak cohesion predicted for such bicrystals, whereas Coulombic interactions are found to play only a minor role. It then follows that (i) the (001) plane is not a favored plane for the formation of grain boundaries in pure oxides with NaCl structure and (ii) similarities should exist between (001) twist boundaries in ionic crystals with NaCl structure and in fee metals. These similarities are investigated by comparing calculated boundary structures and energy-vs-misfit angle curves for MgO with recent results for aluminum, copper, silver, and gold, in which a broad spectrum of different interatomic potentials was used. It is suggested that the rather strong cohesion of the pressure-sintered MgO bicrystals of Sun and Balluffi may be due to impurities which have migrated to the boundary during sintering.     

Sequential Precipitation of MgAl2O4 on Mg1-x CaxAl2O4 in Hot-Pressed MgO Single Crystals

In a hot-pressed and deformed MgO single crystal, precipitates of Mg_1-xCa_xAl₂O₄ spinel upon which MgAl₂O₄ spinel subsequently precipitated were observed and analyzed using transmission electron microscopy and scanning electron microscopy. This behavior is related to the respective solubility limits of CaO and Al₂O₃ in MgO at the hot-pressing temperature and may be aided by impurity segregation to the dislocations. The spinel selectively precipitated at the nodes of a dislocation network which was formed during [001] hot-pressing deformation, as a result of the reaction b₃= b₁+ b₂= (1/2) [011] + (1/2)     = [001]. The dislocation is sessile, and the precipitates have a <100>_matrix≨ <100>_spinel coherent relationship.     

Electrical Conductivity of Pure and Fe-Doped Magnesium-Aluminum Spinel

The electrical conductivity of nominally pure and Fe-doped MgAl₂O₄ spinel was measured at 700 to 2000 K. The results for pure stoichiometric spinel can be fitted by exponential expressions with different activation-energy parameters in three ranges of temperature. The conductivity of a sample containing 0.1% iron is the same as that of pure samples when the iron is in the Fe²⁺ form; it increases approximately a factor of two when the ion is in the Fe³⁺ form. The results are consistent with ionic conduction due to motion of cation vacancies.     

Segregation at Special Boundaries in MgO

Segregation of Ca to three high-angle boundaries in MgO was analyzed using scanning transmission electron microscopy. Two boundaries were special [001] twist boundaries (Σ=5 (Σ=36.9°), Σ=17 (Σ=28.5°)) and one was nonspecial. The concentration of Ca at the special boundaries was approximately half that at the nonspecial boundary.     

Surface Structure in Corundum: II, Dislocation Structure and Fracture of Deformed Single Crystals

The dislocation structure in plastically deformed single crystals of corundum (α-A1₂O₃) was studied by etching and optical techniques. A special method of deformation was used which encouraged the participation of both the basal (0001), (1120) and the prismatic (1210), (1010) slip systems concurrently. This deformation resulted in highly stressed crystals which fractured spontaneously once the outside surface was scratched. Incoherent surface like imperfections, accompanied by distinctive dislocation arrays, were found embedded in the deformed crystals. These "surfaces" appeared as discontinuities, on each side of which dislocations etched differently. A possible origin of observed failure making use of the dislocation reaction ± [1010] = 3=1/3[2110] ± 1/3[1120] is suggested. A mechanism for the observed strain hardening in prismatic bending is presented.     

Yielding of polyethylene through propagation of chain twist defects: Temperature, stem length and strain-rate dependence

The temperature, the stem length and the strain-rate dependence of the yield stress of polyethylene (PE) is investigated via a modified crystal plasticity approach. Yielding is considered in terms of nucleation and propagation of [001] screw dislocations with Burgers vector c/2 due to migration of 180° chain twist defects. The stress-induced twist motion within the dislocation cores is modeled as an Eyring activated rate process. This gives an inelastic contribution to the dislocation core energy depending on the stem length and the strain rate and results in improved predictions of the crystal plasticity approach. The model is compared to available experimental data as well as to the predictions of the modified crystal plasticity approach proposed by Brooks and Mukhtar.     

Grain-Boundary Diffusion of 51Cr in MgO and Cr-Doped MgO

The grain-boundary diffusion product, D'δ , of ⁵¹Cr in MgO and Cr-doped MgO as a function of grain-boundary orientation and point-defect concentration was determined at T =1200° to 1450°C. A large degree of anisotropy was found in the grain-boundary diffusion behavior in MgO. The ratio of D'δ_|| parallel to D'δ_‖ perpendicular to the growth direction, D'_||/D'_‖ , is 10² for a 5° (100) tilt boundary, decreased to ∼2 in boundaries with tilt angles > 10°. The decrease in D'_||/D'_‖ is due to a large increase in D'_‖ with increasing tilt angle. The results indicate that grain-boundary diffusion in MgO is connected to the orientation of dislocations and the mechanism is one of dislocation pipe diffusion. The grain-boundary diffusion product D'δ increases with increasing Cr concentration in MgO and is ∼4 times larger for MgO containing 0.56 at. % Cr than for the undoped MgO. For all bicrystals studied, the activation energies are within 180 ± 20 kJ/mol which is 60% of the activation energy for ⁵¹Cr diffusion in undoped MgO.     

Grain Boundary Migration in Cubic Zirconia–Yttria Induced by Addition of Magnesia at Varying Concentrations

When partially sintered cubic ZrO₂–10 mol% Y₂O₃ specimens are heat-treated at 1500°C with powder mixtures of MgO and ZrO₂–10Y₂O₃ at varying ratios, the grain boundaries migrate, leaving behind new solid solutions enriched with MgO but depleted of Y₂O₃. With increasing MgO content in the solute source powder, the average migration velocity increases, and the MgO content increases and the Y₂O₃ content decreases slightly in these new solid solutions. With increasing MgO content in the solute source, the grain boundaries tend to be corrugated and faceted. Migration reversal is also observed at the corrugated boundaries. These variations of the grain boundary migration behavior with the MgO content in the solute source are consistent with the diffusional coherency strain energy as the driving force for the migration.     

Structural and Optical Characterization of Flower-Like Rutile Nanostructures Doped with Fe3+

Flower-like agglomerates with sizes of 200–400 nm of pure and Fe³⁺-doped TiO₂ with rutile crystalline structure were synthesized by the coprecipitation method. The morphology of the agglomerates was determined by electron microscopy (TEM and HRTEM). TiO₂ agglomerates consist of nanorods with clearly visible crystalline faces, parallel to the axis of elongation whose direction was along the [101] direction of pure TiO₂ and the [111] direction of doped TiO₂. Furthermore, nanorods consist of "chains" of spherical particles, most likely interconnected through the so-called oriented attachment or grain-rotation-induced grain coalescence (GRIGC) process. UV/Vis reflection measurements revealed that the absorption of pure TiO₂ was significantly shifted from UV toward the visible spectral region upon the incorporation of Fe³⁺ into the TiO₂ host.     

Segregation of Aliovalent Solutes Adjacent Surfaces in MgO

The segregation of ferrous iron, ferric iron, chromium, and scandium solutes adjacent {100} surfaces of MgO crystals in the single-phase region was studied by ion microprobe mass spec-troscopy. Excess concentration in a region near the surface was found for Fe³⁺, Cr³⁺, and Sc³⁺, but none for Fe²⁺. This segregation is the type predicted as necessary to balance the negative surface charge.     

The influence of grain boundary misorientation on ionic conductivity in YSZ

Molecular dynamics (MD) was used to investigate the structure and ion transport properties of three interfaces in 8 mol% yttria-stabilized zirconia (YSZ); namely the Σ5 (310)/[001] and Σ13 (320)/[001] tilt grain boundaries and Σ5 (111) 60° twist grain boundary. Atomic interactions were described by a potential function of the Buckingham form. Diffusion rates of oxide ions in the grain boundary containing systems showed that the tilt grain boundaries reduce the overall ionic conductivity relative to a single crystal of 8 mol% YSZ, while the Σ5 twist boundary is able to support rapid diffusion and increases the total conductivity. The effect of segregation of dopant ions to the boundary regions was also investigated.     

Growth of Nickel Oxide Single Crystals and Bicrystals via hemical Vapor Transport

A chemical vapor transport (CVT) method was successfully used to grow NiO single crystals and bicrystals heteroepitaxially on single-crystal MgO substrates. The most favorable growth conditions were obtained at 1400 K using 250 torr (∼3.33 × 10⁴ Pa) of HCl( g ) as the transport agent. Average growth rates greater than 100 μm/h were easily achieved under these conditions. The CVT-grown NiO single crystals and bicrystals usually displayed highly reflective facets along the growth direction that suggest high mechanical quality. The grain boundaries in the bicrystals were observed to be perpendicular to the (001) growth surface. The epitaxial NiO crystals were easily separated from the MgO substrate by dissolving away the latter in 85% H₃PO₄ at 190°C. The crystallinity and purity of the deposits were checked using these free-standing NiO crystals. The concentration of cation impurities and the Cl content in the CVT-grown crystals were investigated by inductively coupled plasma (ICP) mass-spectrometric analysis and neutron activation analysis, respectively. High-resolution transmission electron microscopy of a Σ13 (510) boundary revealed a structure at the atomic scale that provided no evidence for segregated phases.     

Role of Solute Segregation at Grain Boundaries During Final–Stage Sintering of Alumina

The addition of minor amounts of MgO or NiO to Al₂O₃ inhibits grain growth during sintering and allows the sintering process to proceed to theoretical density by maintaining a high diffusion flux of vacancies from the pores to the grain boundaries. The inhibition of grain growth is accomplished by the segregation of solute at the grain boundaries, causing a decrease in the grain–boundary mobility. The segregation of MgO or NiO at the grain boundaries can be inferred from the results of the microhardness studies presented and is substantiated by autoradiographic experiments and also by lattice parameter determinations as a function of grain size.     

Structure-Optical Property Correlation of Epitaxial Potassium Niobate Thin Films Deposited on Magnesium Oxide (100) Substrates Using a Strontium Titanate Transition Layer

Epitaxial thin films of orthorhombic KNbO₃ (100)/(010) were deposited on MgO (100) single-crystal substrates by pulsed laser ablation using an ε120 Å SrTiO₃ (110) transition layer in between. From X-ray diffraction, the orientation relationships were determined as (010) [100] or (100) [010] KNb0₃∥ (110) [110] SrTiO₃∥ (100) [011] MgO and (010) [001] or (100) [001] KNbO₃∥ (110) [001] SrTiO₃∥ (100) [011] MgO. The measured film refractive indices at 632.8 nm were 2.213 ± 0.003, 2.278 ± 0.003, and 2.285 ± 0.003, respectively, along MgO [100], [011], and [011] directions. A model was developed to correlate the measured effective refractive indices of the film to area fractions of domain variants in the film. Using the model, the area fraction of domains with their polarization axis normal to the substrate was estimated to be 60.0 ± 2.7%.     

Influence of Orientation on Properties of Grain Boundaries in NaCl

Simple (100) tilt, twist, and double-tilt bicrystals of NaCl, grown by the Kyropoulos technique from melts of high-purity NaCl, alone and with controlled impurity additions, were examined for mechanical strength and structure of the grain boundaries. Grain boundary fracture strengths, measured in three-point bending, showed that high-purity bicrystals with simple tilt orientations were stronger than those with simple twist at high mismatch angles (37° to 45°). The results did not show a functional dependence of strength on angle of mismatch in either tilt or twist bicrystals. Pips observed on parted grain boundaries of high-purity NaCl (100] twist and double-tilt bicrystals were believed to represent regions of continuity across the boundary. This feature was rare in similar-purity NaCl (100) tilt bicrystals. Separate additions of 100 ppm SiO₂, CaCl₂, FeCls, and KCl to the melt had no apparent effect on the character of the grain boundary. However, an addition of 1000 ppm CaCl₂ nearly doubled the strength of a (1001) 30° twist bicrystal, whereas the same addition weakened a (100) 45° tilt bicrystal. Sodium chloride (100) tilt grain boundaries, examined in situ under dark-field illumination, showed randomly distributed spots, believed to be impurity segregations, with their maximum density at the boundary. The spot densities increased with increasing tilt angle for angles of 15° and greater. The spots were not observed in the low-angle tilt boundaries (<15°) and were seen only in intermittent clusters in the few twist bicrystals examined.     

Precipitate Coarsening by Liquid Film Migration in (Mg,Ca)-PSZ's

Precipitate coarsening involving preferential growth of solute-poor tetragonal ZrO₂ precipitates along grain boundaries, combined with migration of grain boundaries containing a liquid silicate phase to form solute-rich precipitate-free cubic ZrO₂, has been observed in grain boundary regions of MgO–CaO–ZrO₂ and CaO–ZrO₂ alloys during isothermal aging at 1400°C. Studies of alloys with 9.5 mol% total solute, but with varying MgO/CaO ratios, demonstrated a marked composition dependence of the phase separation process, in that such phase separation was rare in MgO-rich materials. The migration process is analyzed using models developed for migration in less complex materials, and a mechanism incorporating orientation-dependent precipitate strain energies is proposed.     

Crystallography of the Tetragonal to Monoclinic Transformation in MgO-Partially-Stabilized Zirconia

The structure of monoclinic ZrO₂ particles dispersed in MgO-partially-stabilized zirconia has been examined systematically using transmission electron microscopy and electron micro-diffraction. In both particles transformed athermally and those transformed under stress, the product of the martensitic tetragonal to monoclinic transformation comprises parallel variants of the monoclinic structure. The monoclinic domains extend either parallel or normal to the original precipitate habit plane and, to within the accuracy of the technique, adjacent pairs are twin related. For particles with domains parallel to the particle habit plane the boundary between variants is (001)_m and the orientation relationship between tetragonal and monoclinic lattices is such that (001)_m‖ (001)_t and [100]_m‖ [100]_t. In particles with transverse domains, the domain boundaries are parallel to (100)_m, and the orientation relationship is given by (100)_m‖ (100)_t and [001]_m‖ [001]_t. In each case the lattice correspondence implied between parent and product lattices is such that the c _m axis is parallel to the C _m axis. The microcracking associated with transformed particles appears closely related to the substructure adopted by the particles and the origin of this microcracking is briefly discussed.